Drilling operations to drill a new wellbore for hydrocarbon extraction, for example, include the common practice of continuously circulating a drilling fluid (alternatively known as a drilling mud) through the wellbore during the drilling operation. The drilling fluid is pumped into the drill pipe to the bottom of the borehole where the drilling fluid then flows upwardly through the annular space between the wellbore wall and the drill pipe, and finally flows from the wellbore where it is recovered for secondary processing. Specifically, the drilling fluid is mechanically or chemically treated to remove captured solids and drill cuttings from the drilling fluid and before recirculation back through the wellbore.
Given the circulating nature of drilling fluid and its functionality in capturing solids and cuttings during drilling operations, drilling fluids must be free-flowing with a relatively low viscosity in order to facilitate pumping while having sufficient substance to retain and transport the cuttings and other solids and to suspend the weighting material so as to maintain a drilling fluid column of uniform density in the wellbore during both static and circulating conditions. The drilling fluid must also have a gel strength sufficient to suspend the solids and cuttings if circulation of the drilling fluid is stopped to prevent accumulation of solids at the bottom of the wellbore. Solids accumulating at the bottom of the wellbore would potentially result in jamming of the drill as well as physical blockage of the drilling fluid's flow path.
Drilling in deep wells is complicated by geological conditions that involve high pressures and high temperatures (HPHT). As wellbores are increased in depth, the pressure and temperature at the base of the wellbore is elevated. The industry-defined definition of HPHT conditions typically include a wellbore temperature greater than 300° F. (149° C.) and a wellbore pressure greater than 10,000 pounds per square inch (psi), approximately 68.9 megapascal (MPa). Elevated temperatures have a detrimental effect upon drilling fluids with breakdown of components unable to sustain the elevated temperatures. At elevated temperatures some drilling fluids may begin to solidify or experience viscosity increases that may impede circulation.